1. Field of the Invention
This invention relates generally to internal combustion engines and the carburetion and fuel systems required therefor. More particularly, this invention relates to improvements in the methods and apparatus for the use of LP-Gas, such as propane, in internal combustion engines used with vehicles. Even more specifically, this new method and apparatus improves the ability of the fuel system to generate a vapor and maintain the requisite vapor pressure in a fuel line through improvements in the tank design.
2. Description of the Prior Art
LP-Gas (liquid petroleum gas) has been available and has been used as a fuel for internal combustion engines since the early 1900's, following the development of propane and butane. LP-Gas vaporizes at a lower temperature than gasoline and is therefore handled differently; it is kept under a higher pressure, drawn from the tank as a liquid, and then vaporized prior to injection into the engine. Since LP-Gas vaporizes at a low temperature this vaporization tendency has created quite a problem; as long as the low temperature and high pressure remain in balance, the fuel will remain a liquid. But when the pressure drops--such as when vapor is withdrawn or when fuel is withdrawn too rapidly--then the remaining fuel boils and causes the temperature to drop, to "freeze" the fuel supply system. To help avoid this difficulty, the technique currently in use in LP-Gas systems is to withdraw the fuel as a liquid, maintain the fuel in the liquid state until reaching the engine vicinity, and then vaporizing the fuel.
Because LPG fuel is clean burning a functional LPG system presents an attractive alternative to gasoline. In a gasoline system it is well known to withdraw the liquid fuel from the tank and then to create a fine mist at the carburetor. Then typically the fuel will substantially, though not entirely, vaporize when entering the intake manifold. In this regard especially, propane has been considered superior because it will vaporize more easily and more completely. The need for cold starting aids is eliminated and it mixes well with air for a cleaner and more efficient combustion. The overall result, to the extent the propane system is feasible, is an environmentally preferred fuel.
Propane fuel tanks differ from gasoline tanks in that they are designed with heavy gauge steel to withstand the high pressures. The usual propane system also employs a pressure relief valve, a manual shut-off valve on the fuel line, a filler valve and guages. Since the fuel system is maintained in a pressurized condition, no fuel pump is used. The key to the prior systems, however, is the vaporizer-regulator. A good discussion of the vaporizer-regulator and the state of the art in LP-Gas fuel delivery systems is provided in the publication LP-GAS CARBURETION available from the National LP-Gas Association (NLPGA) located in Oakbrook, Ill.
The vaporizer-regulator used in LPG systems acts to vaporize the liquid fuel before directing it into the intake manifold. This device is used to avoid injecting the liquid drawn from the tank directly into the engine. Drawing vapor, as an alternative, has been avoided as being unreliable since the vapor pressure varies greatly with the ambient conditions and drops quickly when the vapor is drawn off, thereby rendering the system inoperative. And injecting liquid into the manifold has been avoided since it interferes with the engine operation. The prior art solution is the location of a vaporizer near the carburetor. This vaporizer is composed of a chamber into which the liquid fuel is fed under tank pressure and around which there are provided heating coils. Heated coolant from the engine cooling system is circulated through the coils to heat the chamber. Before entering the vaporizer chamber, however, the fuel is first passed through a pressure regulator to reduce the pressure to a value more acceptable to the engine--typically 10 psi--and such regulator may also employ a vacuum controlled fuel shutoff. Such a regulator is described in U.S. Pat. No. 2,775,981 and is available from Beam Products Mfg., Co.
Under cold operating conditions or high fuel demand requirements, the above described prior art apparatus will encounter a drop in fuel line pressure and a freezing of the fuel lines and of the system. This limit on the operability of the LPG system is partially responsible for its limited popularity; it limits the ambient temperatures in which it can be operated as well as limiting the size of the engine and power available. Moreover, the vaporizer device itself has contributed to the higher expense of the LPG system. Prior to the present invention no apparatus has been proposed for inexpensively vaporizing the LPG and eliminating the vaporizer device. Perhaps most importantly is that no apparatus has been proposed which allows for the high volume withdrawal of vapor from the tank without the risk of a critical pressure drop and a freeze up of the system.